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United States Patent |
6,013,236
|
Takahashi
,   et al.
|
January 11, 2000
|
Wafer
Abstract
A wafer employing a silicon carbide sintered body is provided. The density
of the silicon carbide sintered body is 2.9 g/cm.sup.3 or more. The
silicon carbide sintered body is obtained by sintering a mixture in which
silicon carbide powder and a non-metal-based sintering additive are mixed
uniformly. The non-metal-based sintering additive is formed of an organic
compound which generates carbon upon heating or the like. As a result, a
wafer with excellent heat resistance and acid resistance and which causes
little contamination is provided.
Inventors:
|
Takahashi; Yoshitomo (Fujisawa, JP);
Wada; Hiroaki (Kawasaki, JP);
Miyamoto; Taro (Yokohama, JP)
|
Assignee:
|
Bridgestone Corporation (Tokyo, JP)
|
Appl. No.:
|
941067 |
Filed:
|
September 30, 1997 |
Foreign Application Priority Data
| Oct 03, 1996[JP] | 8-263039 |
| Aug 27, 1997[JP] | 9-231469 |
Current U.S. Class: |
423/345; 264/625 |
Intern'l Class: |
C01B 031/36 |
Field of Search: |
423/345
264/29.1,63,65,56,66,669,664,625
501/88
|
References Cited
U.S. Patent Documents
4788018 | Nov., 1988 | Yamada et al. | 264/63.
|
5298467 | Mar., 1994 | Hurtado et al. | 423/345.
|
5589116 | Dec., 1996 | Kojima et al. | 264/65.
|
5679153 | Oct., 1997 | Dmitriev et al. | 117/106.
|
5863325 | Jan., 1999 | Kanemoto et al. | 423/345.
|
Primary Examiner: Griffin; Steven P.
Assistant Examiner: Hendrickson; Stuart L.
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
What is claimed is:
1. A wafer consisting essentially of a silicon carbide sintered body, said
silicon carbide sintered body having a density of at least 2.9 g/cm.sup.3
and being obtained by sintering a mixture in which silicon carbide powder
and a non-metal based sintering additive are mixed uniformly, such that
said silicon carbide sintered body consists essentially of silicon,
carbon, nitrogen and oxygen, and the total content of impurity elements in
said silicon carbide sintered body is 1 ppm or less.
2. A wafer according to claim 1, wherein said non-metal-based sintering
additive is an organic compound which generates carbon upon heating.
3. A wafer according to claim 1, wherein the surface of said silicon
carbide powder is coated with said non-metal-based sintering additive.
4. A wafer according to claim 2, wherein the surface of said silicon
carbide powder is coated with said non-metal-based sintering additive.
5. A wafer according to claim 1, wherein said silicon carbide sintered body
is obtained by hot-pressing said mixture in a non-oxidizing atmosphere.
6. A wafer according to claim 2, wherein said silicon carbide sintered body
is obtained by hot-pressing said mixture in a non-oxidizing atmosphere.
7. A wafer according to claim 3, wherein said silicon carbide sintered body
is obtained by hot-pressing said mixture in a non-oxidizing atmosphere.
8. A wafer according to claim 4, wherein said silicon carbide sintered body
is obtained by hot-pressing said mixture in a non-oxidizing atmosphere.
9. A wafer according to claim 1, wherein said silicon carbide powder is
obtained by a manufacturing method which includes a solidifying process
and a sintering process; in the solidifying process, a silicon source
containing at least one liquid silicon compound, a carbon source
containing at least one liquid organic compound which generates carbon
upon heating, and one of a polymerizing and a cross-linking catalyst is
mixed and then solidified so as to obtain a solid material; in the
sintering process, the obtained solid material is carbonized through
heating in a non-oxidizing atmosphere, and thereafter, sintered in a
non-oxidizing atmosphere.
10. A wafer according to claim 2, wherein said silicon carbide powder is
obtained by a manufacturing method which includes a solidifying process
and a sintering process; in the solidifying process, a silicon source
containing at least one liquid silicon compound, a carbon source
containing at least one liquid organic compound which generates carbon
upon heating, and one of a polymerizing and a cross-linking catalyst is
mixed and then solidified so as to obtain a solid material; in the
sintering process, the obtained solid material is carbonized through
heating in a non-oxidizing atmosphere, and thereafter, sintered in a
non-oxidizing atmosphere.
11. A wafer according to claim 4, wherein said silicon carbide powder is
obtained by a manufacturing method which includes a solidifying process
and a sintering process; in the solidifying process, a silicon source
containing at least one liquid silicon compound, a carbon source
containing at least one liquid organic compound which generates carbon
upon heating, and one of a polymerizing and a cross-linking catalyst is
mixed and then solidified so as to obtain a solid material; in the
sintering process, the obtained solid material is carbonized through
heating in a non-oxidizing atmosphere, and thereafter, sintered in a
non-oxidizing atmosphere.
12. A wafer consisting essentially of a silicon carbide sintered body, the
silicon carbide sintered body having a density of at least 2.9 g/cm.sup.3
and a total content of impurity elements including boron of 1 ppm or less.
13. A wafer consisting essentially of a silicon carbide sintered body the
silicon carbide sintered body having a density of at least 2.9 g/cm.sup.3
and having a total content of impurity elements selected from the group
consisting of elements of groups 1 to 16 of the periodic table and having
an atomic number of 3 or more, exclusive of carbon, nitrogen, oxygen and
silicon of 1 ppm or less.
14. A method of making a wafer consisting essentially of a silicon carbide
sintered body, comprising:
uniformly mixing silicon carbide powder and a non-metal based sintering
additive to form a mixture;
sintering the mixture to obtain a silicon carbide sintered body that has a
density of at least 2.9 g/cm.sup.3 and that consists essentially of
silicon, carbon, nitrogen and oxygen and has a total content of impurity
elements of 1 ppm or less; and
forming a wafer from the silicon carbide sintered body.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wafer which is used for evaluating the
uniformity of a furnace temperature, concentration of gases, or the like,
for removing contaminants, and for determining various processing
conditions in processing stages of thermal diffusion, thermal oxidation,
and vapor phase epitaxy or the like to a silicon wafer (hereinafter,
"dummy wafer") when an integrated circuit or the like is manufactured.
2. Description of the Related Art
In the manufacturing process of a semiconductor device such as LSI, it is
important to carry out a stage of oxidizing the surface of a wafer, a
stage of diffusing a doping element such as phosphorus or boron within
silicon, a stage of forming various coatings on the surface of the wafer
due to CVD (chemical vapor deposition) or PVD (physical vapor deposition).
In order to improve the yield of a product and to manufacture a device of
higher integrity, it is important to maintain the processing conditions
constant in these stages.
In the above-described stages, in general, a batch processing is used in
which a boat in which 100 pieces or more of wafers are attached is placed
in a reactor having a heater and the wafers are subjected to processings.
However, in such processing, there are drawbacks in that the temperature
and the concentration of raw material gas vary in accordance with the
position of the wafer within the reactor. For this reason, dummy wafers,
which are not used as product wafers, are disposed at places within the
reactor where the processing conditions, such as the temperature within
the reactor and the concentration of the raw material gas may be different
from those at the other places within the reactor. The uniformity of the
processing conditions is assessed by determining whether the thicknesses
and components of thin films, which are superposed one on top of another
at each of the dummy wafers, are the same. Further, the dummy wafers are
used for studying the processing conditions of plasma in an etching device
and for removing particles generated within the device. The dummy wafers
used for these purposes are repeatedly used at high temperatures, and are
repeatedly processed by acid so as to remove the coating formed on the
dummy wafers and thereby enable the repetition of use thereof.
Conventionally, silicon or quartz which is the same as the material of an
ordinary product wafer is used as the material of a dummy wafer. However,
in a case in which a dummy wafer formed with silicon is used, since the
heat resistance of silicon is not good, the configuration of the dummy
wafer is easily changed over time, and since the acid resistance of
silicon is low, the surface of the dummy wafer becomes rough due to
dissolution by acid and particles are easily generated. Accordingly, the
life of the dummy wafer is short. On the other hand, in case of quartz,
since the heat resistance and acid resistance thereof are not sufficient
and quartz is not conductive, the dummy wafer cannot be used for etching
processing or the like. Therefore, instead of silicon and quartz, a carbon
material of excellent heat resistance and a ceramics material of excellent
acid resistance are desirable as the material of dummy wafers. Above all,
since the components are harmless to a semiconductor device serving as a
product, a silicon carbide sintered body is the most desirable.
However, because it is difficult to sinter silicon carbide, a small amount
of boron carbide, alumina, or the like is generally added to the silicon
carbide as an additive for facilitating the sintering. Since these
additives become impurities, a conventional silicon carbide is
inappropriate as a material of the aforementioned dummy wafer.
Accordingly, a silicon carbide sintering method and a sintered body which
does not use the aforementioned harmful additive are desired. For example,
i) a method of manufacturing a sintered body with fine powder formed
through vapor phase epitaxy employing gas or a solution including silicon
and carbon as a material and by using the formed powder as a material; and
ii) a method of manufacturing directly a plate-shaped molded body
(sintered body) through vapor phase epitaxy employing gas or a solution
including silicon and carbon as a material are proposed.
However, in these methods, there are drawbacks in that the productivity is
very poor and the cost is high. Further, the above-described Method i) has
a drawback in that the powder is too fine and particles are easily
generated even after the powder is sintered. Method ii) has a drawback in
that it is difficult to manufacture a thick molded body.
SUMMARY OF THE INVENTION
The present invention was developed in view of the aforementioned
drawbacks, and the object thereof is to provide a dummy wafer which has
good heat resistance and acid resistance and which results in little
contamination.
As a result of assiduous studies, the inventors have found that, when a
sintered body made of silicon carbide obtained through a specific
manufacturing method is used as a dummy wafer, extremely good
characteristics are obtainable. The present invention has been thereby
completed.
Namely, a wafer which is formed by a silicon carbide sintered body, the
density of the silicon carbide sintered body being at least 2.9
g/cm.sup.3, and the silicon carbide sintered body being obtained by
sintering a mixture in which silicon carbide powder and a non-metal-based
sintering additive are mixed uniformly.
In accordance with the present invention, when the silicon carbide powder
is sintered, instead of the combination of a metallic sintering additive
such as a metal (for example, boron, aluminum, and beryllium), and a
compound thereof, and a carbon-based sintering additive such as carbon
black and graphite, only non-metal-based sintering additive, which will be
described later, is used as a sintering additive. Accordingly, a dummy
wafer is provided in which the purity of the sintered body is high, the
number of foreign objects in the crystal boundary is small, thermal
conductivity is good and, as a result, heat resistance is good, and the
fundamental properties of silicon carbide, i.e., good heat resistance,
good acid resistance and low contamination, are better compared to those
of carbon materials.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described in detail hereinafter.
Silicon carbide powder used as a raw material of the dummy wafer of the
present invention manufactured from silicon carbide may be .alpha.-type,
.beta.-type, amorphous silicon carbide powders or mixtures thereof. In
particular, from the viewpoint of coefficient of thermal expansion of the
sintered body, .beta.-type silicon carbide powder is preferably used. The
grade of the .beta.-type silicon carbide powder is not particularly
specified, and for example, .beta.-type silicon carbide powder which is
commercially available can be used. From the viewpoint of densification,
it is preferable that the particle diameter of the silicon carbide powder
be small and from about 0.01 to about 5 .mu.m, and more preferably from
about 0.05 to about 3 .mu.m. If the particle diameter is less than 0.01
.mu.m, it is not preferable since it is difficult to handle the silicon
carbide powder during the measuring and mixing processes and the like. If
the particle diameter exceeds 5 .mu.m, it is not preferable since the
specific surface area of the silicon carbide powder is small, i.e., the
surface area thereof which makes contact with adjacent granules becomes
small, and it is difficult to achieve densification.
As a preferable aspect of silicon carbide raw material powder, a silicon
carbide powder in which the particle diameter is 0.05 to 1 .mu.m, the
specific surface area is 5 m.sup.2 /g or more, the free carbon is 1% or
less, and the oxygen content is 1% or less is preferably used. Further,
the particle size distribution of the silicon carbide powder to be used is
not particularly limited, and from the viewpoints of improving the filling
density of the powder and the reactivity of silicon carbide at the time of
manufacturing the silicon carbide sintered body, silicon carbide powder
having two or more size distribution peaks may be used.
The purity of the silicon carbide sintered body used for the dummy wafer is
preferably high. In order to obtain a silicon carbide sintered body of
high purity, the highly pure silicon carbide powder may be used as the raw
material silicon carbide powder.
The highly pure silicon carbide powder can be obtained, for example, in
accordance with a manufacturing method including a sintering process in
which a solid material obtained by uniformly mixing a silicon source
containing at least one type or more of a liquid silicon compound, a
carbon source containing at least one type or more of a liquid organic
compound which generates carbon upon heating, and a polymerizing or
cross-linking catalyst, is sintered in a non-oxidizing atmosphere.
As a silicon compound (hereinafter occasionally referred to as "silicon
source") which is used in manufacturing the highly pure silicon carbide
powder, the liquid and solid compounds can be combined. However, at least
one type of compound must be selected from the liquid compounds. As the
liquid compound, alkoxysilane (mono-, di-, tri-, tetra-) and polymers of
tetraalkoxysilane is used. Among alkoxylanes, tetraalkoxysilane is
preferably used, and more concretely, methoxysilane, ethoxysilane,
propoxysilane, butoxysilane, or the like can be used. Regarding handling,
ethoxysilane is preferable. Moreover, as the polymer of tetraalkoxysilane,
low molecular weight polymer (oligomer) which has a polymerization degree
of about 2 to 15 and a liquid silicic acid polymer whose polymerization
degree is even higher can be used. As a solid compound which can be
combined, silicon oxide can be used. In the present invention, silicon
oxide includes SiO, silica sol (colloidal hyperfine silica containing
liquid, including an OH group or an alkoxyl group), silicon dioxide
(silica gel, hyperfine silica, quartz powder), or the like.
Among the silicon sources, from the viewpoint of good uniformity and good
handling ability, the oligomer of tetraetoxysilane or the mixture of
oligomer of tetraetoxysilane and fine powder silica is preferable.
Further, the highly pure materials are used for these silicon sources, and
the initial content of impurity is preferably 20 ppm or less, and more
preferably 5 ppm or less.
Further, as an organic compound which is used for manufacturing the silicon
carbide powder of high purity and which generates carbon upon heating, a
liquid compound or a combination of liquid and solid compounds can be
used. It is preferable that an organic compound whose remaining carbon
ratio is high and which is polymerized or cross-linked by a catalyst or
upon heating is, for example, a monomer or prepolymer of a resin such as
phenol resin, furan resin, polyimide, polyurethane, polyvinyl alcohol, or
the like. Other liquid compounds such as cellulose, sucrose, pitch, tar,
or the like may be used, and especially, resol-type phenol resin is
preferable. Further, the purity of the compound can be controlled and
selected appropriately in accordance with purpose. In particular, in the
case in which highly pure silicon carbide powder is needed, it is
preferable to use an organic compound which does not contain 5 ppm or more
of each metal.
When the highly pure silicon carbide powder which is a raw material powder
and is used in the present invention is manufactured, the ratio of carbon
to silicon (hereinafter, "C/Si ratio") is defined by effecting elemental
analysis of a carbide intermediate product obtained by carbonizing a
mixture at 1000.degree. C. Stoichiometrically, when C/Si ratio is 3.0,
free carbon within the generated silicon carbide is 0%. However, in
reality, due to the vaporization of SiO gas which is generated
simultaneously, free carbon generates in a low C/Si ratio. It is important
to determine the blending ratio in advance such that the amount of the
free carbon in the silicon carbide powder produced is a suitable amount
for the production of the sintered body. Normally, in the sintering in the
vicinity of 1 atmospheric pressure and at 1600.degree. C. or more, when
the C/Si ratio is 2.0 to 2.5, the free carbon can be prevented and the
range can be used preferably. When the C/Si ratio is 2.5 or more, the free
carbon increases remarkably. However, because the free carbon has an
effect of preventing particle growth, the range may be selected
appropriately for the purpose of forming particles. In the case where
sintering is performed when the atmospheric pressure is low or high,
because the C/Si ratio for obtaining pure silicon carbide varies, the C/Si
ratio is not necessarily limited to the above range.
Because an effect on sintering which effect free carbon has is very weak
compared to that of the carbon generated by non-metal-based sintering
additive coated on the surface of the silicon carbide powder used in the
present invention, it can be basically ignored.
Moreover, in the present invention, in order to obtain a solid material in
which a silicon source and an organic compound, which generates carbon
upon heating, are mixed uniformly, the mixture of the silicon source and
the organic compound may be hardened, as occasion demands, so as to form a
solid material. As the hardening method, a cross-linking method upon
heating, a hardening method through a hardening catalyst, a method using
electron beams or radiation can be used. The hardening catalyst can be
selected appropriately in accordance with a carbon source. In the case in
which the carbon source is phenol resin or furan resin, acid groups such
as toluenesulfonic acid, toluenecarboxylic acid, acetic acid, oxalic acid,
hydrocholoric acid, sulfuric acid, or the like or amine groups such as
hexamine or the like is used as the hardening catalyst.
The solid material of the raw material mixture may be carbonized through
heating, if necessary. This is performed by heating the solid material at
800.degree. C. to 1000 C for 30 to 120 minutes in a non-oxidizing
atmosphere of nitrogen, argon or the like.
Further, silicon carbide is generated by heating the carbide at a
temperature of from 1350.degree. C. to 2000.degree. C. in a non-oxidizing
atmosphere of argon or the like. The temperature and time for sintering
can be selected appropriately in accordance with the desired
characteristics such as particle diameter and the like. However, for more
effective production, sintering at 1600.degree. C. to 1900.degree. C. is
preferable.
Furthermore, when powder with even higher purity is required, the heating
process is effected for 5 to 20 minutes at 2000 to 2100.degree. C. at the
time of the aforementioned sintering. In this way, impurities can be
further removed.
From the above description, as a method of obtaining silicon carbide powder
whose purity is particularly high, a method of manufacturing a raw
material powder described in a method of manufacturing single crystals,
which was applied as Japanese Patent Application No. 7-241856, can be
used. Namely, a method of manufacturing the highly pure silicon carbide
powder includes a process for generating silicon carbide and a process for
post-treatment. In the process for generating silicon carbide, one or more
compounds selected from the group consisting of highly pure
tetraalkoxysilane, tetraalkoxysilane polymer, and oxidized silicon forms a
silicon source, highly pure organic compound which generates carbon upon
heating forms a carbon source, the sources are mixed uniformly and the
obtained mixture is heated/sintered in a non-oxidizing atmosphere so as to
obtain silicon carbide powder. In the process for post-treatment, the
obtained silicon carbide powder is held at a temperature of 1700.degree.
C. or more to less than 2000.degree. C., and while the powder is held at
this temperature, at least one heating process is effected at 2000.degree.
C. or more to less than 2100.degree. C. for 5 to 20 minutes. By effecting
the above two processes, the silicon carbide powder containing impurities
at 0.5 ppm or less is obtained.
Further, when a silicon carbide sintered body which is used suitably for a
wafer of the present invention is manufactured, as a non-metal-based
sintering additive which is mixed with the above silicon carbide powder, a
material which is a so-called carbon source and generates carbon upon
heating is used. Examples thereof include organic compounds which generate
carbon upon heating or silicon carbide powder (the particle diameter is
about 0.01 to 1 .mu.m) which is coated with the organic compounds. From
the viewpoint of effects, the former is more preferable.
Further, in the present invention, a material which is used as an organic
compound (hereinafter, "carbon source") which is mixed with the
above-described silicon carbide powder and generates carbon upon heating
has a function of promoting reaction by adding as a non-metal-based
sintering additive, instead of conventional sintering additive. Specific
examples of the organic compounds that generate carbon upon heating
include coal tar pitch, phenol resins, furan resins, epoxy resins, phenoxy
resins, and various saccharides including monosaccharides such as glucose,
oligosaccharides such as sucrose, and polysaccharides such as cellulose
and starch, having a high residual carbon ration. For the purpose of
uniform mixing with the silicon carbide powder, it is preferable to use
organic compounds which are liquids at room temperature, organic compounds
which can dissolve in solvents, or organic compounds which are softened or
melted upon heating such as thermoplastic or thermally melting materials.
Above all, phenol resins with which a molded product of a high strength
can be obtained, particularly resol-type phenol resins, are desirable.
When these organic compounds are heated, it is assumed that inorganic
carbon-based compounds such as carbon black or graphite are generated on
the surface (in the vicinity of the surface) of a particle and function
effectively as sintering auxiliaries which remove efficiently the surface
oxidized film of the silicon carbide during the sintering. Even if carbon
black or graphite powder, which are conventionally known as carbon
sintering auxiliaries, are added, the effects of the present invention
which are obtained by adding the above non-metal-based sintering
auxiliaries cannot be achieved.
In the present invention, when the mixture of silicon carbide powder and
non-metal-based sintering auxiliaries is obtained, it is preferable that
the non-metal-based sintering auxiliaries are dissolved or dispersed in a
solvent and that the obtained solution or dispersion is mixed with the
silicon carbide powder. The solvent which is suitable for a compound to be
used as non-metal-based sintering additive is preferable. Concretely, for
phenol resin which is an organic compound which generates carbon upon
suitable heating, lower alcohols such as ethyl alcohol or the like, ethyl
ether, acetone or the like can be selected. Further, regarding these
non-metal-based sintering auxiliaries and the solvent as well, it is
preferable to use those having a low impurity content.
When the amount of the non-metal-based sintering additive mixed with the
silicon carbide powder is too small, the density of the sintered body does
not increase. When the amount of addition thereof is too large, free
carbons increase and hinder densification. The amount depends on the type
of non-metal-based sintering auxiliaries used, but in general, the amount
added is adjusted to 10% by weight or less, and preferably 2 to 5% by
weight. The amount of addition can be determined by determining the amount
of silica (silicon oxide) on the surface of the silicon carbide powder
with a hydrofluoric acid and by calculating stoichiometrically the added
amount of non-metal-based sintering additive which amount is sufficient
for the reduction of silica.
The amount of addition in the form of carbon is determined in consideration
of the residual carbon ratio of the non-metal-based sintering auxiliaries
after thermal decomposition of the auxiliaries (the proportion of carbon
generated within the non-metal-based sintering auxiliaries), based on the
premise that the silica quantified by the above method is reduced by the
carbon generated from the non-metal-based sintering auxiliaries according
to the following reaction formula.
SiO.sub.2 +3C.fwdarw.SiC+2CO
Further, in the silicon carbide sintered body relating to the present
invention, it is preferable that the sum of carbon atoms derived from
silicon carbide included in the silicon carbide sintered body and carbon
atoms derived from non-metal-based sintering auxiliaries exceeds 30% by
weight and is 40% by weight or less. If the content is 30% by weight or
less, the proportion of impurities in the sintered body increases. If the
content exceeds 40% by weight, the carbon content increases, the density
of the obtained sintered body drops, and characteristics of the sintered
body such as strength and oxidation resistance deteriorate, which is not
preferable.
When the silicon carbide sintered body relating to the present invention is
manufactured, at first, silicon carbide powder and non-metal-based
sintering auxiliaries are mixed uniformly. As mentioned before, phenol
resin which is a non-metal-based sintering additive is dissolved in a
solvent such as ethyl alcohol and then the obtained solution is well mixed
with the silicon carbide powder. The mixing can be carried out by known
mixing means, e.g., a mixer, a planetary ball mill, or the like. The
mixing is preferably carried out over 10 to 30 hours, and more
particularly over 16 to 24 hours. After sufficient mixing, the solvent is
removed at a temperature suited to its physical properties, e.g., 50 to
60.degree. C. in the case of the previously-mentioned ethyl alcohol. The
mixture is dried by evaporation. The raw material powder derived from the
mixture is obtained by sieving. From the viewpoint of purification, it is
necessary that a ball mill container and a ball are made of a synthetic
resin containing as little metal as possible. Moreover, a granulating
device such as a spray drier or the like may be used for drying.
The sintering process which is required in the method of manufacturing a
dummy wafer of the present invention is a process in which the mixture of
powder or the molded body of the mixture of powder obtained by a molding
process, to be described later, is placed and hot-pressed within a mold at
a temperature of 2000 to 2400.degree. C., at a pressure of 300 to 700
kgf/cm.sup.2, and in a non-oxidizing atmosphere.
From the viewpoint of the purity of the obtained sintered body, in order
that the molded body does not directly contact the metal portion of the
mold, it is preferable that a graphite material and the like is used on
some or all of the mold, which is used herein, or that a Teflon sheet or
the like is interposed.
In the present invention, the pressure of hot pressing can be 300 to 700
kgf/cm.sup.2. In the case in which pressure is 400kgf/cm.sup.2 or more,
there is the need to select parts for hot pressing, e.g., dies, punches,
or the like, to be used herein, with high pressure resistance.
Here, the sintering process will be described in detail. Before the
hot-pressing process for manufacturing a sintered body, it is preferable
that heating and temperature-raising are carried out under the following
conditions so as to sufficiently remove impurities and to realize full
carbonization of the carbon source. Then, it is preferable that
hot-pressing under the above conditions is performed.
Namely, the following two-stage temperature-raising process is preferably
effected. Firstly, the interior of a furnace is heated slowly from room
temperature to 700.degree. C. in a vacuum. If it is difficult to control
the temperature within the high temperature furnace, temperature can be
raised continuously to 700.degree. C. Preferably, the interior of the
furnace is set to 10.sup.-4 torr, and the temperature is raised slowly
from room temperature to 200.degree. C. and held thereat for a fixed time.
Thereafter, the temperature is continuously raised slowly to 700.degree.
C. and kept there for a fixed time. In the first temperature-raising
process, absorbed water or bonding agents are broken up/decomposed, and
the carbon source is carbonized by thermal decomposition. A suitable
period of time over which the temperature is held at around 200.degree. C.
or at around 700.degree. C. is selected in accordance with the type of
bonding agent and the size of the sintered body. The point in time when
the lowering in the degree of the vacuum becomes small to some extent, is
taken as an indication of whether the holding time is sufficient or not.
If rapid heating is effected at this stage, it is not preferable since the
removal of impurities or carbonization of the carbon source cannot be
effected sufficiently and cracking or holes may appear in the molded body.
For example, regarding a sample of about 5 to 10 g, the interior of the
furnace is set to 10.sup.-4 torr, the temperature is raised slowly from
room temperature to 200.degree. C. and is held thereat for about 30
minutes. Thereafter, the temperature is continuously raised slowly to
700.degree. C. The time period over which the temperature is raised from
room temperature to 700.degree. C. is about 6 to 10 hours, and preferably
around 8 hours. Further, it is preferable that the temperature is held at
around 700.degree. C. for about 2 to 5 hours.
In a vacuum, the temperature is raised from 700.degree. C. to 1500.degree.
C. over about 6 to 9 hours under the above conditions and is held at
1500.degree. C. over about 1 to 5 hours. In this process, it is thought
that silicon dioxide and silicon oxide are reduced. In order to remove
oxygen attached to the silicon, it is important to fully complete
reduction. It is necessary that the temperature is held at 1500.degree. C.
until the generation of carbon monoxide, which is a by-product of the
reduction, is completed, i.e., the decrease in the degree of vacuum
becomes small and the degree of vacuum is recovered to a degree in the
vicinity of 1300.degree. C.; the temperature before reduction. By the
reducing reaction in the second temperature-raising process, silicon
dioxide, which adheres to the surface of the silicon carbide powder,
prevents densification and causes the growth of oversized particles, is
removed. Gas which includes SiO and CO generated during the reducing
reaction includes impurity elements. However, the gas generated is
constantly discharged into a reaction furnace and removed with a vacuum
pump. Accordingly, from the viewpoint of purification, it is preferable
that the temperature is held sufficiently.
After the temperature-raising process is finished, high-pressure hot
pressing is preferably carried out. When the temperature is raised above
1500.degree. C., sintering starts. At that time, in order to prevent
abnormal particle growth, pressure is applied at 300 to 700 kgf/cm.sup.2.
Thereafter, in order to make the interior of the furnace a non-oxidizing
atmosphere, inert gas is introduced thereto. As the inert gas, nitrogen or
argon is used. Since argon is not reactive at high temperatures, argon gas
is preferably used.
After the interior of the furnace is made non-oxidizing, heating and
pressure-application is effected so that the temperature is 2000 to
2400.degree. C. and the pressure is 300 to 700 kgf/cm.sup.2. The pressure
at the time of pressing can be selected according to the particle diameter
of the raw material powder. When the particle diameter of the raw material
powder is small, even if the pressure at the time of application is
relatively small, a suitable sintered body is obtained. Further, the
temperature is raised from 1500.degree. C. to 2000-2400.degree. C., which
is the maximum temperature, over 2 to 4 hours. Sintering advances rapidly
at 1850 to 1900.degree. C. Further, this maximum temperature is held for 1
to 3 hours and then sintering is completed.
When the maximum temperature is less than 2000.degree. C., it is not
preferable since densification is unsatisfactory. When the temperature
exceeds 2400.degree. C., it is not preferable because there is a risk that
the molded body raw material gets sublimated (decomposed). Further, when
the pressure applied is less than 500 kgf/cm.sup.2, it is not preferable
since densification is unsatisfactory. When the condition exceeds 700
kgf/cm.sup.2, molds such as graphite molds may be broken, and thus, it is
not preferable as regards the manufacturing efficiency.
In the sintering process, from the viewpoint of maintaining the purity of
the obtained sintered body, the graphite mold, the thermal insulating
material of the heating furnace, or the like used herein is preferably a
highly pure graphite raw material. The graphite raw material which was
subjected to high purity processing is used. Concretely, graphite raw
material baked sufficiently beforehand at a temperature of 2500.degree. C.
or more which material does not generate impurities at the sintering
temperature is desirable. Further, it is preferable that the inert gas
used is a high purity product with few impurities.
In the present invention, through the above-described sintering process,
the silicon carbide sintered body having excellent characteristics can be
obtained. From the viewpoint of densification of the finally-obtained
sintered body, a molding process, which will be described later, may be
effected in advance of the sintering process. The molding process which
can be performed in advance of the sintering process is described herein.
The molding process is the step in which raw material powder obtained by
mixing uniformly silicon carbide powder and a carbon source, is placed in
a mold, and then heated and pressurized within the temperature range of 80
to 300.degree. C. for 5 to 60 minutes and in which the molded body is
adjusted beforehand. From the viewpoint of densification of the final
sintered body, it is preferable that the mold is filled with the raw
material powder extremely densely. This molding step allows to make bulky
powder compact previously in filling the sample for the hot pressing.
Accordingly, this facilitates production of a highly dense sintered body
and a thick sintered body by repeating this molding step.
While the heating temperature is within the range of 80 to 300.degree. C.,
and preferably 120 to 140.degree. C. and the pressure is within the range
of 60 to 100 kgf/cm.sup.2, the filled raw material powder is pressed so
that the density thereof is 1.5 g/cm.sup.3 or more, and preferably 1.9
g/cm.sup.3 or more and held in a pressurized state for 5 to 60 minutes,
preferably 20 to 40 minutes. The molded body formed by the raw material
powder is thereby obtained. As the average particle diameter of the powder
is decreased, it is more difficult to increase the density of the molded
body. For densification, it is preferable to use vibration packing for
placing the powder in the mold. More specifically, the density is
preferably 1.8 g/cm.sup.3 or higher for the powder having the average
particle diameter of about 1 .mu.m, and 1.5 g/cm.sup.3 or higher for the
powder having the average particle diameter of about 0.5 .mu.m. If the
density of the particle diameter is less than 1.5 g/cm.sup.3 or less than
1.8 g/cm.sup.3, respectively, densification becomes a problem in the
density of the finally-obtained sintered body.
Before the sintering process to follow, it is possible for the molded body
to be cut beforehand to fit into the hot press mold used. The molded body
is disposed in a mold at 2000 to 2400.degree. C., and at a pressure of 300
to 700 kgf/cm.sup.2, in a non-oxidizing atmosphere. The molded body is
then subjected to a process for hot pressing, i.e., a sintering process. A
silicon carbide sintered body having high density and purity is thereby
obtained.
A silicon carbide sintered body generated as described above is
sufficiently densified and the density thereof is 2.9 g/cm.sup.3 or more.
If the density of the obtained sintered body is less than 2.9 g/cm.sup.3,
it is not preferable because mechanical characteristics such as bending
strength, fracture strength or the like, and electrical physical qualities
are lowered, and additionally, particle numbers are increased and
contaminating effect worsens. More preferably, the density of the silicon
carbide sintered body is 3.0 g/cm.sup.3 or more.
Further, if the obtained sintered body is a porous body, there are physical
drawbacks in that heat resistance, oxidization resistance, chemical
resistance and mechanical strength are poor, cleaning is difficult, minute
cracking occurs and minute pieces become contaminants, and gas permeation
occurs. Thus, the problem of application limitation becomes significant.
The total content of impure elements in the silicon carbide sintered body
obtained in the present invention is 5 ppm or less, preferably 3 ppm or
less, and more preferably 1 ppm or less. As far as application in the
industrial field of semiconductors is concerned, the impurity content as
defined through chemical analysis is merely a reference value. In
practice, evaluation depends on whether the impurity is dispersed
uniformly or locally. Therefore, those skilled in the art use a practical
device in general and evaluate by various means the extent to which the
impurity contaminates a wafer under predetermined heating conditions. In
accordance with a method including a sintering process in which a solid
material (which is obtained by uniformly mixing a liquid silicon compound,
a liquid organic compound which generates carbon upon heating, and
polymerizing or cross-linking catalyst) is carbonized through heating in a
non-oxidizing atmosphere, and thereafter, sintered in a non-oxidizing
atmosphere. The total content of the impurity element contained in the
silicon carbide sintered body can be as low as 1 ppm or less. Further, at
this time, it is necessary that the above raw material selects a substance
of appropriate impurity in accordance with the desirable impurity of the
obtained silicon carbide sintered body. Here, the impurity elements are
elements of groups 1 to 16 of the periodic table described in the IUPAC
Inorganic Chemistry Nomenclature Revised Edition (1989), and having an
atomic number of 3 or more, except for atomic numbers 6 to 8 and 14.
In addition, preferable physical properties of a silicon carbide sintered
body obtained in the present invention are taken into consideration. For
example, it is preferable that bending strength at room temperature is
50.0 to 65.0 kgf/mm.sup.2, bending strength at 1500.degree. C. is 55.0 to
80.0 kgf/mm.sup.2, Young's modulus is 3.5.times.10.sup.4 to
4.5.times.10.sup.4, Vickers hardness is 2000 kgf/mm.sup.2 or more,
Poisson's ratio is 0.14 to 0.21, the coefficient of thermal expansion is
3.8.times.10.sup.-6 to 4.2.times.10.sup.-6 (.degree. C..sup.-1), the
thermal conductivity is 150 W/m.multidot.K or more, the specific heat is
0.15 to 0.18 cal/g.multidot..degree. C., the thermal shock resistance is
500 to 700 .DELTA.T.degree. C., and the specific resistance is 1
.OMEGA..multidot.cm or less.
The sintered body obtained in accordance with the above manufacturing
method is subjected to machining, polishing, washing treatments or the
like for the purpose of actual application. The dummy wafer of the present
invention can be manufactured by means of forming a cylindrical sample
(sintered body) by hot pressing or the like and of slicing the sample in a
radial direction thereof. Electrical discharge machining is preferably
used as the slicing process.
In the present invention, as an example, a dummy wafer which has a diameter
of 100 to 400 mm and a thickness of 0.5 to 1.0 mm can be manufactured.
Moreover, in accordance with its application, the surface roughness of the
wafer, which is the arithmetic average roughness (Ra), can be adjusted to
a range of 0.01 to 10 .mu.m by polishing.
In the above manufacturing method, as long as the above heating conditions
are satisfied, the manufacturing device or the like is not particularly
limited. If the pressure resistance of the mold for sintering is taken in
account, well-known heating furnaces or reaction devices can be used.
The contents of impurity elements is preferably 5 ppm or less in each of a
silicon carbide powder which is the raw material powder of the present
invention, a silicon source and a carbon source for manufacturing the raw
material powder, and an inert gas which is used to produce the
non-oxidizing atmosphere. However, as long as the purity is within the
permissible range of purification during the heating and sintering
processes, the purity is not necessarily limited to this value. Moreover,
impurity elements are elements of groups 1 to 16 of the periodic table
described in the IUPAC Inorganic Chemistry Nomenclature Revised Edition
(1989), and having an atomic number 3 or more, except for atomic numbers 6
to 8 and 14.
EXAMPLES
The present invention will be described hereinafter concretely by giving
Examples. As long as the scope of the present invention is not exceeded,
the present invention is not limited to these Examples.
Example 1
Manufacture of Molded Body
141 g of commercially available .beta.-type silicon carbide powder (Grade
B-HP, manufactured by H. C. Starck Co., of average particle diameter 2
.mu.m) and a solution in which 9 g of highly pure liquid resol-type phenol
resin having a water content of 20% were dissolved in 200 g of ethanol,
were stirred in a planetary ball mill for 18 hours until mixed fully.
Thereafter, the mixture was dried off by evaporating the ethanol at 50 to
60.degree. C. The mixture was put through a 500 .mu.m sieve and uniform
silicon carbide raw material powder was obtained. 15 g of raw material
powder was put into a mold and pressed at 130.degree. C. for 20 minutes.
Accordingly, a cylindrical molded body having a density of 2.2
g./cm.sup.3, an outer diameter of about 200 mm, and a thickness of about
100 mm was obtained.
Manufacture of Sintered Body
The molded body was placed in a graphite mold and subjected to hot pressing
under the following conditions. As a device for hot pressing, a high
frequency induction heating-type 100t hot press was used.
(Conditions of Sintering Process)
In a vacuum state of 10.sup.-5 to 10.sup.-4 torr, the temperature was
raised from room temperature to 700.degree. C. over 6 hours and held
thereat for 5 hours (the first temperature-raising process).
In the vacuum state, the temperature was raised from 700.degree. C. to
1200.degree. C. over 3 hours, and was further raised from 1200.degree. C.
to 1500.degree. C. over 3 hours and held thereat for an hour (the second
temperature-raising process).
Moreover, pressure of 500 kgf/cm.sup.2 was applied, and the temperature was
raised from 1500.degree. C. to 2200.degree. C. over 3 hours in an argon
atmosphere and held thereat for an hour (hot-pressing process).
The density of the obtained sintered body was 3.18 g/cm.sup.3, the Vickers
hardness thereof was 2500 kgf/mm.sup.2, and the electrical specific
resistance thereof was 0.3 .OMEGA..multidot.cm. The obtained sintered body
was thermally decomposed by acid in a heat processing, and thereafter, was
evaluated by ICP-mass analysis and flameless atomic absorption. The
results are given in Table 1.
Manufacture of Dummy Wafer
The sintered body obtained as described above was cut by an electrical
discharge machine and the cut surface was further polished by a polishing
machine. Consequently, a dummy wafer having a diameter of 200 mm and a
thickness of 0.6 mm was obtained.
Example 2
Manufacture of Highly Pure Silicon Carbide Powder
680 g of a highly pure ethyl silicate oligomer having a silica content of
40% and 305 g of a highly pure liquid resol-type phenol resin having a
water content of 20% were mixed. As catalyst, 137 g of a highly pure
toluenesulfonic acid 28% aqueous solution was added to the mixture and
hardened, so that a uniform resinous solid material was obtained. The
solid material was carbonized in a nitrogen atmosphere at 900.degree. C.
for an hour. From the result of elemental analysis, the proportion of C to
Si of the obtained carbide was 2.4. Next, 400 g of the carbide was placed
in a graphite container, the temperature was raised to 1850.degree. C. in
an argon atmosphere and held thereat for 10 minutes. Thereafter, the
temperature was raised to 2050.degree. C. and held thereat for 5 minutes.
Then, the temperature was lowered and powder having an average particle
diameter of 1.3 .mu.m was obtained. The level of impurities of each
element was 0.5 ppm or less.
Manufacture of Molded Body
141 g of highly pure silicon carbide powder, which was obtained in
accordance with the above-described method, and a solution in which 9 g of
a highly pure liquid resol-type phenol resin having a water content of 20%
were dissolved in 200 g of ethanol, were stirred in a planetary ball mill
for 18 hours until mixed fully. Thereafter, the mixture was dried off by
evaporating the ethanol at 50 to 60.degree. C. The mixture was put through
a 500 .mu.m sieve and uniform silicon carbide raw material powder was
obtained. 15 g of raw material powder was put into a mold and pressed at
130.degree. C. for 20 minutes. Accordingly, a cylindrical molded body
having a density of 2.1 g/cm.sup.3, an outer diameter of about 200 mm, and
a thickness of about 100 mm was obtained.
Manufacture of Sintered Body
The molded body was placed in a graphite mold and subjected to hot pressing
under the following conditions. As a device for hot pressing, a high
frequency induction heating-type 100t hot press was used.
(Conditions of Sintering Process)
In a vacuum state of 10.sup.-5 to 10.sup.-4 torr, the temperature was
raised from room temperature to 700.degree. C. over 6 hours and held
thereat for 5 hours (the first temperature-raising process).
In the vacuum state, the temperature was raised from 700.degree. C. to
1200.degree. C. over 3 hours, and was further raised from 1200.degree. C.
to 1500.degree. C. over 3 hours and held thereat for an hour (the second
temperature-raising process).
Moreover, pressure of 500 kgf/cm.sup.2 was applied, and the temperature was
raised from 1500.degree. C. to 2200.degree. C. over 3 hours in an argon
atmosphere and held thereat for an hour (hot-pressing process).
The density of the obtained sintered body was 3.15 g./cm.sup.3, the Vickers
hardness thereof was 2600 kgf/mm.sup.2, and the electrical specific
resistance thereof was 0.2 .OMEGA..multidot.cm. The impurity
concentrations are shown in the following Table 1.
Further, as a result of measuring in detail the physical properties of the
obtained sintered body in Example 2, characteristics other than those
given above were as follows: the bending strength at room temperature was
50.0 kgf/mm.sup.2 ; the bending strength at 1500.degree. C. was 50.0
kgf/mm.sup.2 ; the Young's modulus was 4.1.times.10.sup.4 ; the Poisson's
ratio was 0.15; the coefficient of thermal expansion is
3.9.times.10.sup.-6 .degree. C..sup.-1 ; the thermal conductivity was 200
W/m.multidot.K or more; the specific heat was 0.16 cal/g.multidot..degree.
C.; and the thermal shock resistance was 530 .DELTA.T.degree. C. It was
confirmed that all of the aforementioned desirable physical properties
were satisfied.
Manufacture of Dummy Wafer
The sintered body obtained as described above was cut by an electrical
discharge machine and the cut surface was further polished by a polishing
machine. Consequently, a dummy wafer having a diameter of 200 mm and a
thickness of 0.6 mm was obtained.
Comparative Example 1
Manufacture of Molded body
141 g of commercially available .beta.-type silicon carbide powder (Grade
B-HP, manufactured by H. C. Starck Co., of average particle diameter 2
.mu.m), 1.1 g of boron carbide (B.sub.4 C) and a solution in which 9 g of
a highly pure liquid resol-type phenol resin having a water content of 20%
were dissolved in 200 g of ethanol, were stirred in a planetary ball mill
for 18 hours until mixed fully. Thereafter, the mixture was dried off by
evaporating the ethanol at 50 to 60.degree. C. The mixture was put through
a 500 .mu.m sieve and uniform silicon carbide raw material powder was
obtained. 15 g of raw material powder was put into a mold and pressed at
130.degree. C. for 20 minutes. Accordingly, a cylindrical molded body
having a density of 2.2 g/cm.sup.3, an outer diameter of about 200 mm, and
a thickness of about 100 mm was obtained.
Manufacture of Sintered Body
The molded body was placed in a graphite mold and subjected to hot pressing
under the following conditions. As a device for hot pressing, a high
frequency induction heating-type 100t hot press was used.
(Conditions of Sintering Process)
In a vacuum state of 10.sup.-5 to 10.sup.-4 torr, the temperature was
raised from room temperature to 700.degree. C. over 6 hours and held
thereat for 5 hours (the first temperature-raising process).
In the vacuum state, the temperature was raised from 700.degree. C. to
1200.degree. C. over 3 hours, and was further raised from 1200.degree. C.
to 1500.degree. C. over 3 hours and held thereat for an hour (the second
temperature-raising process).
Moreover, pressure of 150 kgf/cm.sup.2 was applied, and the temperature was
raised from 1500.degree. C. to 2200.degree. C. over 3 hours in an argon
atmosphere and held thereat for an hour (hot-pressing process).
The density of the obtained sintered body was 3.18 g/cm.sup.3, the Vickers
hardness thereof was 2400 kgf/mm.sup.2, and the electrical specific
resistance thereof was 10.sup.8 .OMEGA..multidot.cm. The impurity
concentrations are shown in the following Table 1.
Manufacture of Dummy Wafer
The sintered body obtained as described above was cut by an electrical
discharge machine and the cut surface was further polished by a polishing
machine. Consequently, a dummy wafer having a diameter of 200 mm and a
thickness of 0.6 mm was obtained.
Comparative Example 2
A commercially available highly pure graphite dummy wafer (density 1.65
g/cm.sup.3, Vickers hardness 350 kgf/mm.sup.2, electrical specific
resistance 2.4.times.10.sup.-3 .OMEGA..multidot.cm) was used.
The impurity concentrations are shown in the following Table 1.
Comparative Example 3
A commercially available dummy wafer (density 2.33 g/cm.sup.3, Vickers
hardness 550 kgf/mm.sup.2, electrical specific resistance
1.3.times.10.sup.-2 .OMEGA..multidot.cm) was used.
The impurity concentrations are shown in the following Table 1.
TABLE 1
______________________________________
Impurity Concentrations of Sintered Bodies (ppm)
Comparative
Comparative
Comparative
Example 1 Example 2
Example 1 Example 2
Example 3
______________________________________
B 0.8 0.00 1000 or 0.05 <0.01
more
Al 48 0.02
55
0.03
0.012
Na 3 0.03
10
0.12
<0.01
K 1.2
0.00
1.0
0.06
<0.01
Mg 5 0.05
4 <0.01
Ti 2 0.02
5 <0.01
Cr 5 0.00
7 <0.01
Fe 33 0.03
48
0.08
<0.01
Ni 4 0.01
5 <0.01
Co 4 0.03
3 <0.01
W 1.2
0.00
1.0
0.00
<0.01
Cu 0.5 0.00
0.8
0.02
<0.01
______________________________________
The evaluation was made on the heat resistance, the contaminating effect
and acid resistance of dummy wafers of the above Examples and Comparative
Examples. The methods of evaluation are given as follows.
Heat Resistance
Five sheets of dummy wafers of each of Examples and Comparative Examples
were disposed at a wafer boat, and then the wafer boat was mounted to a
diffusion device which enabled batch processing. The temperature within
the device was increased to 1250.degree. C., held thereat for 10 hours,
and thereafter, cooled to a room temperature. The degree of deformation
(%) of a dummy wafer after the cycle was repeated 10 times [(the amount of
in-plane sinuosity of dummy wafer before the test)/(the amount of in-plane
sinuosity of dummy wafer after the test).times.100](%) was obtained. The
amount of in-plane sinuosity herein was obtained by the average value of
the two: the difference of altitude between the highest portion and the
lowest portion of the first virtual cut surface when it was assumed that
the wafer was cut in the thick direction thereof along the first line
(radius) which passed through the center of the wafer; and in the same
way, the difference of altitude between the highest portion and the lowest
portion of the second virtual cut surface when it was assumed that the
wafer was cut in the thick direction thereof along the second line
(radius) which passed through the center of the wafer and was orthogonal
to the first line.
Contaminating Effect
Two sheets of dummy wafers of each of Examples and Comparative Examples
were disposed at a wafer boat so as to nip a silicon wafer. Subsequently,
the wafer boat was mounted to a diffusion device which enabled batch
processing. The temperature within the device was increased to
1250.degree. C., held thereat for 10 hours, and thereafter, cooled to a
room temperature. Then, the number of iron atoms within 1 .mu.m of the
wafer surface was measured.
Acid Resistance
Five sheets of dummy wafers of each of the Examples and Comparative
Examples were disposed at a wafer boat, and then the wafer boat was
mounted to a CVD film formation device which enabled batch processing. The
temperature within the device was increased to 800.degree. C., and
monosilane gas and oxygen were introduced into the device so as to effect
coating formation for about 10 hours. Thereafter, the respective dummy
wafers were taken out and subjected to washing which dissolves silicon
oxide layer in a strong acid containing mainly hydrofluoric acid. The
weight loss (%) of dummy wafer after the operation was effected 10 times
[1-(the weight of dummy wafer after the test)/(the weight of dummy wafer
before the test).times.100] was obtained.
The evaluation results are given in Table 2.
TABLE 2
______________________________________
Compar- Compar-
Compar-
ative ative ative
Example 1 Example 2
Example 1
Example 2
Example 3
______________________________________
Heat 112 103 108 125 340
Resistance
(%)
Contamina-
6.5 .times. 10.sup.13
1.8 .times. 10.sup.10
7.4 .times. 10.sup.15
2.9 .times. 10.sup.10
1.3 .times. 10.sup.10
ting Effect
(atoms/cm.sup.2)
Acid 0.8 0.5 1.2 15 35
Resistance
(%)
______________________________________
As can be seen from the above Examples and Comparative Examples, the
silicon carbide sintered bodies of Examples obtained in accordance with
the methods of the present invention had adequate density, extremely low
impurity content, and excellent heat resistance and acid resistance.
Further, in the Examples, the contaminating effect on the wafer thereof
was low.
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